The well-known Bayer process is widely used for the production of alumina from alumina-containing ores such as bauxite. In this process, alumina-containing ores are contacted with recycled caustic aluminate liquor at elevated temperatures. This produces a slurry of material which comprises dissolved (alumina-containing) and undissolved red mud (iron oxides, silicates, titanium oxide, etc.) components. The undissolved residues are removed, the liquor cooled and aluminum hydroxide is added to the remaining clear solution (which is known as "pregnant liquor"). The added aluminum hydroxide acts as a seed for the further precipitation of aluminum hydroxide in this solution. The precipitated aluminum hydroxide is then separated from the caustic aluminate solution. A portion of the aluminum hydroxide is recycled to be used as a seeding agent for further precipitation of aluminum hydroxide and the remainder is recovered as product. The remaining caustic aluminate solution (hereinafter referred to as "spent liquor") is recycled in the process for further alumina recovery as it is, or is evaporated prior to the first extraction step.
The bauxite used in the Bayer process contains organic substances which wholly or partly dissolve during the bauxite digestion step. Under the influence of the high caustic concentration and elevated temperatures during bauxite digestion, the organic contaminants degrade to lower molecular weight compounds. In the Bayer liquor, therefore, the organic contaminants comprise a wide spectrum of molecules ranging from complex high molecular weight compounds to final degradation products, such as sodium oxalate and sodium carbonate.
The organic contaminants of the Bayer process inhibit the precipitation of aluminum hydroxide, reduce liquor productivity and generally lower the purity of the produced alumina. Therefore, it is necessary to lower the organic carbon concentration of the Bayer plant liquor. This can be accomplished by accelerating the oxidizion of the organic carbon contaminant of the liquor to the final products sodium oxalate or, preferably, sodium carbonate.
A method for this purpose is known from U.S. Pat. No. 4,215,094 to Inao, et al., which uses temperatures up to 300.degree. C. and copper ions (in the form of cupric sulfate) of at least 100 mg/l as well as oxygen in at least stoichiometric quantities required to oxidize the organic contaminants of the Bayer liquor. Sodium sulfide is used to precipitate and recover the copper ions from oxidized liquor as copper sulfide. However, the copper sulfide thus precipitated is not easy to separate by filtration for recycling.
In this prior art, it is claimed that if the organics oxidation efficiency is limited to 60-85%, whereby a sufficient concentration of crystalline sodium oxalate is maintained in the oxidized liquor, the sodium oxalate-copper sulfide mixture is filterable. In order to recycle the catalyst, however, the sodium oxalate crystals must be removed in a second oxidation step.
Furthermore, unless a considerable excess of Na.sub.2 S is used to precipitate copper sulfide, copper ions remain in solution and can enter the Bayer process liquor and contaminate the product alumina. Sulfate ions are also introduced in significant amounts unless preventive measures are taken to remove them.
Therefore, what is needed in the art is an improved method for the oxidative degradation of organic carbon contaminants in Bayer liquors whereby oxidation efficiencies of greater than 85% can be achieved without the above-mentioned difficulties in filtration and without the introduction of sulfate ions into the works liquor.
The present inventors have discovered a new method for achieving the aforementioned objective. This is done by contacting the caustic aluminate liquor, which contains the organic substances, with molecular oxygen-containing gas at elevated pressure and in the presence of metallic ions acting as catalysts. This produces a substantially complete oxidation of the organic carbon to sodium carbonate. The oxidative degradation of organic substances leads to a lower organic soda concentration in the Bayer liquor and, therefore, a reduced soda and organic carbon concentration in the product alumina. Moreover, it is well known that the removal of said organic contaminants leads to a substantial improvement in the alumina yield.